Separation of gases

ABSTRACT

Aspects of the disclosure relate to the separation of gases and to a process for the removal of carbon dioxide gas using liquid absorbents. A process is disclosed for removing carbon dioxide from a gaseous stream comprising contacting the gaseous stream with a carbon dioxide absorbent comprising a mixture of an ionic liquid and water in a molar ratio of from 10:1 to 1:10, wherein the ionic liquid has the formula: [Cat+][X−].

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. §119(b) of UKPatent Application No. 1116062.9, filed Sep. 16, 2011, and titled“SEPARATION OF GASES.” UK Patent Application No. 1116062.9 is hereinincorporated by reference in its entirety.

BACKGROUND

The separation of carbon dioxide from gas streams is an active field ofresearch due to the increasing concern about global warming from thegreenhouse effect and the common belief that the build-up of carbondioxide in the atmosphere is a contributing factor.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key and/oressential features of the claimed subject matter. Also, this Summary isnot intended to limit the scope of the claimed subject matter in anymanner.

Aspects of the disclosure relate to the separation of gases and to aprocess for the removal of carbon dioxide gas using liquid absorbents.

DRAWINGS

FIG. 1 is an example graph showing the CO₂ uptake of the carbon dioxideabsorbents;

FIG. 2 is a histogram showing a comparison of the CO₂ absorption ofvarious CO₂ absorbents from Examples 3 to 7.

DETAILED DESCRIPTION

Aspects of the disclosure are described more fully hereinafter withreference to the accompanying drawings, which form a part hereof, andwhich show, by way of illustration, example features. The features can,however, be embodied in many different forms and should not be construedas limited to the combinations set forth herein; rather, thesecombinations are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope. The following detaileddescription is, therefore, not to be taken in a limiting sense.

The separation of carbon dioxide from gas streams is an extremely activefield of research, due to the increasing concern about global warmingfrom the greenhouse effect, and the common belief that the build-up ofcarbon dioxide in the atmosphere is a contributing factor.

In the field of natural gas processing, separation of carbon dioxide andother acidic (sour) gases from the natural gas stream is also ofimportance to improve the fuel quality and to avoid corrosion problemsin metal pipelines.

Separation of carbon dioxide is also important in the field of biogas.Biogas is the product of microbial degradation of organic waste. Biogasstreams contain large amounts of carbon dioxide (and other acid gases)and water vapour, in addition to the methane and light hydrocarbons ofinterest. Hence, removal of carbon dioxide to improve the quality ofbiogas is necessary as for natural gas.

Carbon dioxide removal processes are of importance in the field of lifesupport systems. For human-occupied sealed systems containing abreathing gas mixture, such as submarines and SCBA gear as well asunderground mining shelters and spacecraft, the build-up of metabolitessuch as carbon dioxide and water vapour must be removed to maintainlife. Current technologies rely on carbon dioxide scrubbers (typically asolid material such as lithium hydroxide or zeolites) as well as aseparate dehumidifier (see for example BioPak 240). As is the case fornatural gas and biogas treatment, the process may be simplified bycombing the two procedures in one step.

For all of these reasons and others, the implementation of carbondioxide separation processes in industry has gained significantly inimportance over the last few decades.

The technologies used for the purpose of carbon dioxide separation canbe divided into three groups: liquid absorbents, solid absorbents oradsorbents, and membranes. Liquid absorbents are by far the mostcommonly used of these, and can in turn be divided into physical andchemical absorbents.

In the field of natural gas processing, chemical absorbents are oftenpreferred, since they generally have higher absorption capacities forcarbon dioxide. The energy efficiency of chemical absorption processesis determined largely by the energy demands of regenerating theabsorbent by desorption of the carbon dioxide.

The most commonly used chemical absorbents for carbon dioxide removalinclude aqueous amine solutions, chilled ammonia and hot aqueouspotassium carbonate. However, these require high temperatures to berecycled, in general far above 100° C. In addition, the high basicityand solvent strength of these systems often leads to other problems suchas corrosion of pipelines and other steel parts in the plant. Anotherdrawback of these liquid chemical absorption systems, in particular theaqueous amine systems, is that the amines are volatile and toxic andthus can contaminate the gas stream with loss of absorbent, and alsorepresent a hazard in the workplace. With hot aqueous potassiumcarbonate absorbents, the major drawbacks include high operatingtemperatures and the tendency of the salts to crystallize/precipitateduring processing. Dilution of the solutions so as to minimize corrosionor crystallization/precipitation issues entails a consequent reductionin carbon dioxide absorption capacity.

Ionic liquids are a class of compounds which have been the subject ofintense research over the past few decades. The term “ionic liquid” asused herein refers to a liquid that is capable of being produced bymelting a solid, and when so produced consists solely of ions. The term“ionic liquid” includes both compounds having high melting temperatureand compounds having low melting points, e.g. at or below roomtemperature (i.e. 15 to 30° C.). The latter are often referred to as“room temperature ionic liquids” and are often derived from organicsalts having pyridinium- and imidazolium-based cations. A feature ofionic liquids is that they have particularly low (essentially zero)vapour pressures. Many organic ionic liquids have low melting points,for example, less than 100° C., particularly less than 80° C., andaround room temperature, e.g. 15 to 30° C., and some have melting pointswell below 0° C.

An ionic liquid may be formed from a homogeneous substance comprisingone species of cation and one species of anion, or it can be composed ofmore than one species of cation and/or anion. Thus, an ionic liquid maybe composed of more than one species of cation and one species of anion.An ionic liquid may further be composed of one species of cation, andmore than one species of anion.

Ionic liquids generally exhibit a set of useful physicochemicalcharacteristics that typically include extremely low vapour pressure,wide liquid range, non-degradability, non-flammability, good thermalstability and excellent ability to solubilise a large range ofcompounds. Due to the potential for controlling these properties ofionic liquids by judicious choice of the constituent ions, and the largevariety of ions that can be combined to form low-melting salts, ionicliquids have been proposed for a broad range of applications.

Ionic liquids have been proposed as an alternative to chemical andphysical acid gas absorbents for a number of reasons including: (i) thepossibility of controlling their properties by the selection of thecation and anion components; (ii) the limited tendency of ionic liquidsto crystallize under operating conditions; and (iii) the potential toprevent contamination of the gaseous streams by the absorbent due to thenegligible vapour pressure of ionic liquids.

Anderson et al. (Accounts of Chemical Research, 2007, volume 40, pages1208 to 1216) have reviewed the absorption of a number of differentgases in pyridinium, imidazolium and ammonium ionic liquids. The molarenthalpies (ΔH) of gas dissolution were determined for the group ofgases tested, and the low values observed indicate that only physicalabsorption takes place. In particular, carbon dioxide is said tointeract with the ionic liquids by means of dispersion, dipole/induceddipole interactions and electrostatic effects.

The use of ionic liquids as chemical CO₂ absorbers has also beenreported. Bates et al. (Journal of the American Chemical Society, 2002,volume 124, pages 926 to 927) have reported the use of a basicimidazolium ionic liquid having an amine functionality tethered to theimidazolium cation to sequester carbon dioxide as a carbamate. However,the high viscosity of these ionic liquids both before, and especiallyafter, carbon dioxide sequestration is a serious limitation for theirpotential use in industrial processes.

Carvalho et al. (Journal of Physical Chemistry B, 2009, volume 113,pages 6803 to 6812) have reported the use of 1-butyl-3-methylimidazoliumionic liquids having acetate and trifluoroacetate anions as absorbentsfor carbon dioxide. This document teaches that purifying the ionicliquid by removal of water prior to use is essential to avoid areduction in carbon dioxide absorbing capacity which is reported to takeplace when water is present in the ionic liquid. A number of prior artdocuments are cited by Carvalho et al., each of which support thedeleterious effect of using wet ionic liquids for carbon dioxideabsorption.

The absorption of carbon dioxide by ionic liquids containing imidazoliumcations is also disclosed by Shiflett et al. (Journal of PhysicalChemistry B, 2008, volume 112, pages 16654 to 16663). Again, the ionicliquids are purified by removing water under vacuum with heating for aperiod of 5 days, emphasizing the need for the ionic liquids to be dry.A single phosphonium ionic liquid (tetra-n-butylphosphonium formate) wasalso analysed, again in the absence of water, and shown to absorb modestamounts of carbon dioxide by a physical absorption mechanism.

The present inventors have found, however, that the use of imidazoliumionic liquids in industrial processes is seriously limited by theinstability of these ionic liquids to many of the components found innatural gas streams. Reaction of imidazolium ions with components of thegaseous stream can lead to loss of absorbent, and contamination of theremaining absorbent with degradation products of the ionic liquid. Theinstability of imidazolium ionic liquids has been discussed in detail byAggarwal, V. K. et. al. (Chemical Communications 2002, 1612-1613) andEarle, M. J. at the ACS symposium Washington D.C. 2001 (Abstracts ofPapers of the American Chemical Society, 2001, volume 221, 161). Thereis therefore a need in the art for alternative ionic liquid absorbentsthat are both capable of absorbing useful quantities of carbon dioxidefrom gaseous streams, while also being resistant to degradation duringprocessing.

The present disclosure is based on the surprising discovery that,contrary to the teaching in the art to rigorously dry ionic liquids thatare used to absorb carbon dioxide, selected classes of ionic liquidsdemonstrate a marked improvement in carbon dioxide absorption capacityin the presence of water. More specifically, it has surprisingly beenfound that mixtures of water and ionic liquids having anions that areconjugate bases of acids having a pKa of at least 3.60, in factdemonstrate a marked improvement in carbon dioxide absorption capacitywhen compared to the ionic liquid alone in the absence of water.

In addition, it has been found that the selected ionic liquids areextremely stable to the processing conditions used for separation ofcarbon dioxide from gaseous streams, and are highly resistant todegradation by other contaminants that may be present in the gaseousstreams.

It has also been found that mixtures of the selected classes of ionicliquids and water are also highly effective in removing othersubstances, such as water vapour, from gaseous streams. The mixtures ofionic liquids and water may therefore be used in processes for thecombined removal of carbon dioxide and one or more additionalsubstances, such as water, from gaseous streams.

Furthermore, it has been found that carbon dioxide and other substancescan be easily desorbed from the mixtures of ionic liquids and water. Forinstance, carbon dioxide can be desorbed by heating in the presence of asparging gas (e.g. nitrogen gas), or by pressure reduction, allowing themixtures to be recycled to the separation process without loss ordegradation of the absorbent mixture.

According to the present disclosure, there is provided a process forremoving carbon dioxide from a gaseous stream comprising contacting thegaseous stream with a carbon dioxide absorbent comprising a mixture ofan ionic liquid and water in a molar ratio of from 10:1 to 1:10, whereinthe ionic liquid has the formula:[Cat⁺][X⁻]

-   -   wherein: [Cat⁺] represents one or more cationic species selected        from tetrasubstituted phosphonium cations, tetrasubstituted        ammonium cations, trisubstituted sulfonium cations, guanidinium        cations and quinolinium cations; and    -   [X⁻] represents one or more anionic species selected from        conjugate bases of acids having a pKa of at least 3.6,    -   with the proviso that [X⁻] is not formate, acetate,        trifluoroacetate, hydroxyacetate, propanoate,        pentafluoro-propanoate, lactate, butanoate, isobutanoate,        pivalate, pyruvate, thiolactate, oxalate, tartrate, malonate,        succinate, adipate or benzoate.

Preferably, [Cat⁺] is selected from tetrasubstituted phosphoniumcations, tetrasubstituted ammonium cations, and trisubstituted sulfoniumcations having the formulae:[P(R^(a))(R^(b))(R^(c))(R^(d))]⁺, [N(R^(a))(R^(b))(R^(c))(R^(d))]⁺ and[S(R^(b))(R^(c))(R^(d))]⁺

-   -   wherein R^(a), R^(b), R^(c), and R^(d) are each independently        selected from a C₁ to C₂₀ straight chain or branched alkyl        group, a C₃ to C₈ cycloalkyl group, or a C₆ to C₁₀ aryl group,        or wherein any two of R^(a), R^(b), R^(c), and R^(d) may        together form a saturated methylene chain of the formula        —(CH₂)_(q)—, where q is an integer of from 4 to 7, or an        oxyalkylene chain of the formula —(CH₂)₂—O—(CH₂)₂—, wherein said        alkyl, cycloalkyl or aryl groups, said methylene chain, or said        oxyalkylene chain are unsubstituted or may be substituted by one        to three groups selected from: C₁ to C₆ alkoxy, C₂ to C₁₂        alkoxyalkoxy, C₆ to C₁₀ aryl, —CN, —OH, —NO₂, —CO₂(C₁ to        C₆)alkyl, —OC(O)(C₁ to C₆)alkyl, C₇ to C₃₀ aralkyl C₇ to C₃₀        alkaryl, and —N(R^(z))₂, where each R^(z) is independently        selected from hydrogen, methyl, ethyl, n-propyl and iso-propyl,        and wherein R^(b) may also be hydrogen.

More preferably, [Cat⁺] is selected from tetrasubstituted phosphoniumcations and tetrasubstituted ammonium cations having the formulae:[P(R^(a))(R^(b))(R^(c))(R^(d))]⁺ and [N(R^(a))(R^(b))(R^(c))(R^(d))]⁺

wherein: R^(a), R^(b), R^(c), and R^(d) as defined above.

Still more preferably, [Cat⁺] is selected from tetrasubstitutedphosphonium cations having the formula:[P(R^(a))(R^(b))(R^(c))(R^(d))]⁺

wherein: R^(a), R^(b), R^(c), and R^(d) as defined above.

-   -   In the tetrasubstituted phosphonium cations, tetrasubstituted        ammonium cations, trisubstituted sulfonium cations defined        above, R^(a), R^(b), R^(c), and R^(d) (where present) are        preferably each independently selected from a C₁ to C₁₆ straight        chain or branched alkyl group, or any two of R^(a), R^(b),        R^(c), and R^(d) may together form a methylene chain of the        formula —(CH₂)_(q)—, where q is an integer of from 4 or 5.

More preferably, R^(a), R^(b), R^(c), and R^(d) (where present) arepreferably each independently selected from a C₁ to C₁₆ straight chainor branched alkyl group. Examples of preferred alkyl groups include:methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl,n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, and n-tetradecyl.

Still more preferably, R^(a), R^(b) and R^(c), (where present) arepreferably each independently selected from a C₁ to C₈ straight chain orbranched alkyl group, and still more preferably a C₁ to C₄ straightchain or branched alkyl group, and R^(d) is preferably a C₁ to C₁₆straight chain or branched alkyl group, and still more preferably a C₁to C₈ straight chain or branched alkyl group.

Still more preferably, R^(a), R^(b) and R^(c), (where present) are eachthe same C₁ to C₈ straight chain or branched alkyl group and mostpreferably the same C₁ to C₄ straight chain or branched alkyl group, andR^(d) is preferably a C₁ to C₁₆ straight chain or branched alkyl group,and most preferably a C₁ to C₈ straight chain or branched alkyl group.

Preferably, R^(d) is different from each of R^(a), R^(b) and R^(c).

In a further preferred embodiment, two of R^(a), R^(b), R^(c), and R^(d)(where present) taken together form a saturated methylene chain of theformula —(CH₂)_(q)—, where q is an integer of from 4 to 7, or anoxyalkylene chain of the formula —(CH₂)₂—O—(CH₂)₂—. Preferably, q is aninteger of 4 or 5.

Examples of preferred tetrasubstituted phosphonium cations andtetrasubstituted ammonium cations and trisubstituted sulfonium cationsin accordance with the present invention, include those where R^(a),R^(b) and R^(c) (where present) are each the same alkyl group selectedfrom ethyl, n-butyl and n-hexyl, and where R^(d) is selected frommethyl, ethyl, n-butyl, n-hexyl, n-octyl, n-decyl, n-dodecyl, andn-tetradecyl.

In embodiments, the tetrasubstituted phosphonium cations andtetrasubstituted ammonium cations used in accordance with the presentinvention are non-symmetrical. As used herein, the term non-symmetricalmeans that at least one of R^(a), R^(b), R^(c), and R^(d) is differentfrom each of the other three of R^(a), R^(b), R^(c), and R^(d). Forexample, preferred non-symmetrical cations include those in which R^(d)is different from each of R^(a), R^(b), and R^(c), wherein R^(a), R^(b),and R^(c) may be the same or different.

Specific examples of phosphonium cations that may be used in accordancewith the present invention include n-butyl-triethylphosphonium,n-hexyl-triethylphosphonium, n-octyl-triethylphosphonium,tetra-n-butylphosphonium, n-hexyl-tri-n-butylphosphonium,n-octyl-tri-n-butylphosphonium, n-decyl-tri-n-butylphosphonium,n-dodecyl-tri-n-butylphosphonium, n-octyl-tri-n-hexylphosphonium,n-decyl-tri-n-hexylphosphonium, n-dodecyl-tri-n-hexylphosphonium, andn-tetradecyl-tri-n-hexylphosphonium.

Particularly preferred phosphonium cations includetetra-n-butylphosphonium and n-octyl-tri-n-butylphosphonium.

Specific examples of ammonium cations that may be used in accordancewith the present invention include tetraethylammonium,n-butyl-triethylammonium, n-hexyl-triethylammonium,n-octyl-triethylammonium, methyl-tri-n-butylammonium,tetra-n-butylammonium, n-hexyl-tri-n-butylammonium,n-octyl-tri-n-butylammonium, n-decyl-tri-n-butylammonium,n-dodecyl-tri-n-butylammonium, n-octyl-tri-n-hexylammonium,n-decyl-tri-n-hexylammonium, n-dodecyl-tri-n-hexylammonium,n-tetradecyl-tri-n-hexylammonium, choline.

Particularly preferred ammonium cations include tetraethylammonium andmethyl-tri-n-butylammonium.

Further examples of cyclic ammonium cations include those wherein two ofR^(a), R^(b), R^(c), and R^(d) (where present) taken together form asaturated methylene chain of the formula —(CH₂)₄— (pyrrolidinium), or ofthe formula —(CH₂)₅— (piperidinium), or an oxyalkylene chain of theformula —(CH₂)₂—O—(CH₂)₂—(morpholinium), and wherein the other two ofR^(a), R^(b), R^(c), and R^(d) (where present) are as defined above.

Where, [Cat⁺] is a quinolinium cation, it preferably has the formula:

-   -   wherein: R^(a), R^(b), R^(c), R^(d), R^(e), R^(f), R^(g), R^(h)        and R^(i) are each independently selected from hydrogen, a C₁ to        C₂₀ straight chain or branched alkyl group, a C₃ to C₈        cycloalkyl group, or a C₆ to C₁₀ aryl group, or any two of        R^(h), R^(c), R^(d), R^(e), R^(f), R^(h) and R^(i) attached to        adjacent carbon atoms may form a saturated methylene chain        —(CH₂)_(q)— wherein q is from 3 to 6, and wherein said alkyl,        cycloalkyl or aryl groups, or said methylene chain are        unsubstituted or may be substituted by one to three groups        selected from: C₁ to C₆ alkoxy, C₂ to C₁₂ alkoxyalkoxy, C₆ to        C₁₀ aryl, —CN, —OH, —NO₂, —CO₂(C₁ to C₆)alkyl, —OC(O)(C₁ to        C₆)alkyl, C₇ to C₃₀ aralkyl C₇ to C₃₀ alkaryl, and —N(R^(z))₂,        where each R^(z) is independently selected from hydrogen,        methyl, ethyl, n-propyl and iso-propyl.

In the above quinolinium cations, R^(a) is preferably selected from C₁to C₂₀ linear or branched alkyl, more preferably C₂ to C₂₀ linear orbranched alkyl, still more preferably C₂ to C₁₆ linear or branchedalkyl, and most preferably C₄ to C₁₀ linear or branched alkyl. Examplesof suitable R^(a) groups include ethyl, butyl, hexyl, octyl, nonyl,decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl,heptadecyl and octadecyl.

In the above quinolinium cations, R^(b), R^(c), R^(d), R^(e), R^(f),R^(h) and R^(i) are preferably independently selected from hydrogen andC₁ to C₅ linear or branched alkyl, and more preferably R^(b), R^(c),R^(d), R^(e), and R^(f) are hydrogen.

Examples of preferred quinolinium and cations which may be used inaccordance with the present invention include:N—(C₈-C₁₈)alkyl-quinolinium, and N—(C₈-C₁₈)alkyl-6-methylquinolinium.

Where, [Cat⁺] is a guanidinium cation, it preferably has the formula:

-   -   wherein: R^(a), R^(b), R^(c), R^(d), R^(e), and R^(f) are each        independently selected from a C₁ to C₂₀ straight chain or        branched alkyl group, a C₃ to C₈ cycloalkyl group, or a C₆ to        C₁₀ aryl group, or any two of R^(b), R^(c), R^(d), R^(e), R^(f),        R^(h) and R^(i) attached to adjacent carbon atoms may form a        saturated methylene chain —(CH₂)_(q)— wherein q is from 3 to 6,        and wherein said alkyl, cycloalkyl or aryl groups, or said        methylene chain are unsubstituted or may be substituted by one        to three groups selected from: C₁ to C₆ alkoxy, C₂ to C₁₂        alkoxyalkoxy, C₆ to C₁₀ aryl, —CN, —OH, —NO₂, —CO₂(C₁ to        C₆)alkyl, —OC(O)(C₁ to C₆)alkyl, C₇ to C₃₀ aralkyl C₇ to C₃₀        alkaryl, and —N(R^(z))₂, where each R^(z) is independently        selected from hydrogen, methyl, ethyl, n-propyl and iso-propyl.

In the above guanidium cations, R^(a) is preferably selected from C₁ toC₂₀ linear or branched alkyl, more preferably C₂ to C₂₀ linear orbranched alkyl, still more preferably C₂ to C₁₆ linear or branchedalkyl, and most preferably C₄ to C₁₀ linear or branched alkyl. Examplesof suitable R^(a) groups include ethyl, butyl, hexyl, octyl, nonyl,decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl,heptadecyl and octadecyl.

In the above guanidinium cations, R^(b), R^(c), R^(d), R^(e) and R^(f)are preferably selected from C₁ to C₁₀ linear or branched alkyl, morepreferably, C₁ to C₅ linear or branched alkyl, and most preferablyR^(b), R^(c), R^(d), R^(e) and R^(f) are each a methyl group.

As noted above, [X⁻] represents an anionic species which is a conjugatebase of an acid having a pKa of 3.6 or more. More preferably, [X⁻]represents an anionic species which is a conjugate base of an acidhaving a pKa of 4.0 or more, still more preferably 5.0 or more, stillmore preferably 6.0 or more, and most preferably 7.0 or more.

Still more preferably, [X⁻] represents an anionic species which is aconjugate base of an acid having a pKa of 15.0 or less, more preferably14.0 or less, more preferably 13.0 or less, still more preferably 12.0or less, still more preferably 11.0 or less and most preferably 10.0 orless.

In accordance with the present disclosure, [X⁻] may represent an anionicspecies which may be a monoanion or a dianion.

In an embodiment, [X⁻] is selected from phosphate dianions andphosphonate dianions.

More preferably, [X⁻] is selected from phosphate anions having theformula [R^(X)OP(O)O₂]²⁻ and phosphonate anions having the formula[R^(x)P(O)O₂]²⁻, wherein R^(x) is selected from hydrogen, a C₁ to C₁₀straight chain or branched alkyl group, a C₃ to C₈ cycloalkyl group, aC₆ to C₁₀ aryl group, a C₆ to C₁₄ aralkyl group, or a C₆ to C₁₄ alkarylgroup, wherein said alkyl, cycloalkyl, aryl, aralkyl, or alkaryl groupsare unsubstituted or may be substituted by one or more groups selectedfrom —F, —Cl, —Br, —I, —OH, —CN, —NO₂, —SH, and ═O.

More preferably, R^(x) is selected from hydrogen or a C₁ to C₁₀ straightchain or branched alkyl group, wherein said alkyl group is optionallysubstituted by one or more groups selected from —F, —Cl, —Br, —I, and—OH.

Still more preferably, R^(x) is selected from hydrogen or a C₁ to C₅straight chain or branched alkyl group, wherein said alkyl group isoptionally substituted by one or more groups selected from —F, —Cl, —Br,—I, and —OH.

Examples of suitable R^(x) groups include hydrogen, methyl, ethyl,n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, fluoromethyl,chloromethyl, bromomethyl, iodomethyl, and hydroxymethyl.

Preferably, [X⁻] is selected from phosphate anions having the formula[R^(X)OP(O)O₂]²⁻, wherein R^(x) is as defined above.

Examples of particularly preferred phosphate anions include[HOP(O)O₂]²⁻, [MeOP(O)O₂]²⁻, [EtOP(O)O₂]²⁻, [n-PrOP(O)O₂]²⁻, and[N-BuOP(O)O₂]²⁻.

Still more preferably, [X⁻] is [HOP(O)O₂]²⁻ (also referred to herein as[HPO₄]²⁻or hydrogen phosphate).

In a further embodiment, [X⁻] is selected from dicarboxylate dianionshaving the formula [O₂C—R^(y)—CO₂]²⁻, wherein R^(y) represents a C₂ toC₆ straight chain or branched alkylene or alkenylene chain, a C₁ to C₆cycloalkylene group, or a C₆ arylene group, wherein said alkylene,alkenylene, cycloalkylene or arylene groups are unsubstituted or may besubstituted with one or more groups selected from —F, —Cl, —Br, —I, —OH,—CN, —NO₂, —NH₂, —SH, —CO₂H, and ═O, with the proviso that [X⁻] is notmalonate, succinate or adipate.

Preferably said alkylene, alkenylene, cycloalkylene or arylene groupsare unsubstituted or substituted with one or more groups selected from—OH, and —SH.

More preferably, [X⁻] may be one or more dicarboxylate dianions selectedfrom glutarate dianion, pimelate dianion, methylmalonate dianion,fumarate dianion, maleate dianion, methyl succinate dianion, malatedianion, citrate dianion, itaconate dianion, mesaconate dianion,o-phthalate dianion, m-phthalate dianion, p-pthalate dianion, aspartatedianion, glutamate dianion, octanedioic acid dianion, and heptanedioicacid dianion.

Still more preferably, [X⁻] may be one or more dicarboxylate dianionsselected from glutarate dianion, pimelate dianion, methylmalonatedianion, fumarate dianion, maleate dianion, methyl succinate dianion,malate dianion, itaconate dianion, and mesaconate dianion.

In a further embodiment, [X⁻] may be selected from ascorbate anion andurate anion.

In a further embodiment, [X⁻] may be selected from heptanedioic acidmonoanion and octanedioic acid monoanion.

In a further embodiment, the present invention provides a process asdefined above, with the proviso that [X⁻] does not comprise a conjugatebase of a monocarboxylic acid.

In a further embodiment, the present invention provides a process asdefined above, with the proviso that [X⁻] does not comprise a conjugatebase of a carboxylic acid.

According to a further aspect, there is provided a process for removingcarbon dioxide from a gaseous stream comprising contacting the gaseousstream with a carbon dioxide absorbent comprising a mixture of an ionicliquid and water in a molar ratio of from 10:1 to 1:10, wherein theionic liquid has the formula:[Cat⁺][X⁻]

-   -   wherein: [Cat⁺] represents one or more cationic species selected        from tetrasubstituted phosphonium cations, tetrasubstituted        ammonium cations, trisubstituted sulfonium cations, guanidinium        cations and quinolinium cations; and    -   [X⁻] represents one or more anionic species selected from        conjugate bases of acids having a pKa of at least 3.6 and        selected from oxalate dianion, tartrate dianion, malonate        dianion, succinate dianion, and adipate dianion.

In accordance with this aspect, [Cat⁺] is preferably as defined above.

According to a further aspect, there is provided a process for removingcarbon dioxide from a gaseous stream comprising contacting the gaseousstream with a carbon dioxide absorbent comprising a mixture of an ionicliquid and water in a molar ratio of from 10:1 to 1:10, wherein theionic liquid has the formula:[Cat⁺][X⁻]

-   -   wherein: [Cat⁺] represents one or more cationic species selected        from tetrasubstituted phosphonium cations, tetrasubstituted        ammonium cations, trisubstituted sulfonium cations, guanidinium        cations and quinolinium cations; and    -   [X⁻] represents one or more anionic species selected from        conjugate bases of acids having a pKa of at least 3.6 and        selected from oxalate monoanion, tartrate monoanion, malonate        monoanion, succinate monoanion, and adipate monoanion.

In accordance with this aspect, [Cat⁺] is preferably as defined above.

In view of the foregoing disclosure, it will be appreciated that it isnot limited to ionic liquids comprising cations and anions having only asingle charge. Thus, the formula [Cat⁺][X⁻] is intended to encompassionic liquids comprising, for example, doubly, triply and quadruplycharged cations and/or anions. The relative stoichiometric amounts of[Cat⁺] and [X⁻] in the ionic liquid are therefore not fixed, but canvary to take account of cations and anions with multiple charges. Forexample, the formula [Cat⁺][X⁻] should be understood to include ionicliquid species having the formulae [Cat⁺]₂[X²⁻]; [Cat²⁺][X⁻]₂;[Cat²⁺][X²⁻]; [Cat⁺]₃[X³⁻]; [Cat³⁺][X⁻]₃ and so on.

The ionic liquids used in accordance with the above aspects preferablyhave a melting point of 200° C. or less, more preferably 150° C. orless, and most preferably 100° C. or less. However, ionic liquids withmelting points falling outside this range may also be used, providedthat the mixture of an ionic liquid and water is liquid at the operatingtemperature of the process.

Thus, in a preferred embodiments of the invention, the mixture of anionic liquid and water has a melting point of 100° C. or less, morepreferably 80° C. or less, more preferably 50° C. or less, still morepreferably 30° C. or less, and most preferably 25° C. or less.

The molar ratio of ionic liquid to water is preferably in the range offrom 5:1 to 1:10, more preferably in the range of from 2:1 to 1:10, morepreferably in the range of from 1:1 to 1:10, still more preferably inthe range of from 1:1 to 1:8, still more preferably in the range of from1:1 to 1:7, and most preferably in the range of from 1:1 to 1:6. Forexample, in preferred embodiments of the invention, the molar ratio ofionic liquid to water may be 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9or 1:10.

The gaseous stream is preferably contacted with the carbon dioxideabsorbent at a temperature of from 10 to 80° C., more preferably from 10to 50° C. and most preferably from 20 to 30° C. For example, the gaseousstream may be contacted with the carbon dioxide absorbent at atemperature at or around 25° C.

The gaseous stream is preferably contacted with the carbon dioxideabsorbent at a pressure of from 100 to 5000 kPa, more preferably from100 to 2000 kPa, and most preferably from 200 to 1000 kPa. For example,the gaseous stream may be contacted with the carbon dioxide absorbent ata pressure at or around 500 kPa.

The processes of the present disclosure may be used to remove carbondioxide, and optionally one or more additional substances, from a numberof different types of gaseous streams. For example, the process may beused to remove carbon dioxide from flue gases. As used herein, the term“flue gas” refers to the exhaust gas from a combustion process, such asthe exhaust gases from furnaces, internal combustion engines and powerplants. Flue gases generally comprise carbon dioxide and water, alongwith other components such as carbon monoxide, nitrogen, nitrogenoxides, and uncombusted fuel components.

The processes may also be used to remove carbon dioxide, and optionallyone or more additional substances, from hydrocarbon-containing gaseousstreams. As used herein, the term “hydrocarbon-containing gaseousstream” refers to a gas containing at least 50 volume percenthydrocarbons, more preferably at least 60 volume percent hydrocarbons,more preferably at least 70 volume percent hydrocarbons, still morepreferably at least 80 volume percent hydrocarbons, still morepreferably at least 90 volume percent hydrocarbons, and most preferablyat least 95 volume percent hydrocarbons. In addition to carbon dioxide,hydrocarbon-containing gaseous streams may contain other gaseouscomponents such as carbon monoxide, nitrogen and water vapour.

In particular, the processes may be used to remove carbon dioxide, andoptionally one or more additional substances, fromhydrocarbon-containing gaseous streams wherein at least 50 volumepercent, more preferably at least 60 volume percent, more preferably atleast 70 volume percent, still more preferably at least 80 volumepercent, still more preferably at least 90 volume percent, and mostpreferably at least 95 volume percent of the hydrocarbons are methane.Thus, the process of the invention may advantageously be used from theremoval of carbon dioxide, and optionally one or more additionalsubstances, from natural gas and/or biogas.

The processes may advantageously be used for the removal of carbondioxide from breathing gas mixtures in life support systems.

Without being bound by any particular theory, it is believed thatabsorption of carbon dioxide by the mixture of ionic liquid and wateroccurs at least in part by the formation of H₂CO₃ (carbonic acid) by thedissolution of carbon dioxide in water, followed by an acid/basereaction with the ionic liquid anion. This hypothesis is supported bythe observation that ionic liquids containing anions which are conjugatebases of acids having a lower pKa value than carbonic acid (3.60) arenot as efficient as absorbers of carbon dioxide. It is believed thatthese ionic liquids are mainly physical absorbers of carbon dioxide andundergo little, if any, chemical interaction with the absorbed carbondioxide.

A further advantage of the ionic liquids is that they are capable ofboth chemical and physical absorption processes. It has been observedthat initial absorption of carbon dioxide by the ionic liquids proceedsby way of chemical absorption. However, once the chemical absorptioncapacity of the ionic liquid is spent, absorption of carbon dioxidecontinues via physical processes. This is observed experimentally by adistinct change in slope of a graph of CO₂ absorption against CO₂partial pressure.

As noted above, carbon dioxide can be easily desorbed from the mixturesof ionic liquids and water by heating in the presence of a sparging gas(e.g. nitrogen gas), and/or by pressure reduction, allowing the mixturesto be recycled to the separation process without loss or degradation ofthe absorbent mixture. It has been found that the mixture of ionicliquid and water can be repeatedly recycled to the separation processwith little or no decrease in the carbon dioxide absorption capacity ofthe absorbent.

It will be appreciated that where the ionic liquid/water absorbent isalso used to remove water from the gas stream, then it will be necessaryto periodically or continuously remove water from the absorbent so as tomaintain the water content of the absorbent within the limits indicatedabove.

Water may be removed from the absorbent for example by drying at 60 to80° C. under vacuum, by sparging a dry gas at elevated temperature, orby reverse osmosis.

Where the process is operated continuously, a portion of the absorbentis continuously or periodically removed from the process to removecarbon dioxide and, if necessary, water so as to restore the compositionof the absorbent. The absorbent is subsequently recycled to the to thecarbon dioxide removal process.

In accordance with the processes, a gaseous stream is recovered havingreduced content of carbon dioxide, and optionally one or more othersubstances such as water, when compared to the composition of thegaseous stream fed to the process.

It will be appreciated that the process may be integrated intoprocessing plants as one stage of a multi-stage processing of gaseousstreams. For instance, the process of the present invention could beused in a natural gas refinery as one stage in the production of acommercial natural gas product, wherein other stages could includeremoval of nitrogen and removal of heavy hydrocarbons. Alternatively,the process of the present invention could be used in a flue gastreatment plant as one stage of a multi-stage processing of flue gases,where other stages could for instance include removal of particulatesand catalytic conversion of NO_(x).

In another aspect, the present disclosure provides the use of a mixtureof an ionic liquid and water in a molar ratio of from 10:1 to 1:10,wherein the ionic liquid has the formula:[Cat⁺][X^(−])

-   -   wherein: [Cat⁺] represent one or more cationic species as        defined above; and    -   [X⁻] represents one or more anionic species selected from        conjugate bases of acids having a pKa of at least 3.6,    -   with the proviso that [X⁻] is not formate, acetate,        trifluoroacetate, hydroxyacetate, propanoate,        pentafluoro-propanoate, lactate, butanoate, isobutanoate,        pivalate, pyruvate, thiolactate, oxalate, tartrate, malonate,        succinate, adipate or benzoate, for the removal of carbon        dioxide from a gaseous stream.

In a further aspect, the present disclosure provides the use of amixture of an ionic liquid and water in a molar ratio of from 10:1 to1:10, wherein the ionic liquid has the formula:Cat⁺][X⁻]

-   -   wherein: [Cat⁺] represent one or more cationic species as        defined above; and    -   [X⁻] represents one or more anionic species selected from        conjugate bases of acids having a pKa of at least 3.6, and        selected from oxalate dianion, tartrate dianion, malonate        dianion, succinate dianion, and adipate dianion, for the removal        of carbon dioxide from a gaseous stream.

In a further aspect, the present disclosure provides the use of amixture of an ionic liquid and water in a molar ratio of from 10:1 to1:10, wherein the ionic liquid has the formula:[Cat⁺][X⁻]

-   -   wherein: [Cat⁺] represent one or more cationic species as        defined above; and    -   [X⁻] represents one or more anionic species selected from        conjugate bases of acids having a pKa of at least 3.6, and        selected from oxalate monoanion, tartrate monoanion, malonate        monoanion, succinate monoanion, and adipate monoanion, for the        removal of carbon dioxide from a gaseous stream.

Preferably, said uses are for the removal of carbon dioxide and at leastone other substance from a gaseous stream. Most preferably, the at leastone other substance is water.

Definitions of the ionic liquid which are said to be preferred inrelation to the processes in the foregoing disclosure are also preferredin accordance with the uses.

In accordance with the uses, said gaseous stream may be ahydrocarbon-containing gaseous stream, for example a methane-containinggaseous stream such as a natural gas stream or a biogas-derived stream.Alternatively, said gaseous stream may be a flue gas stream. In afurther alternative, said gaseous stream may be a breathing gas streamfor a life support system.

The present disclosure will now be described by reference to thefollowing examples, and the attached figures, in which:

-   -   FIG. 1 is a representative graph showing the CO₂ uptake of the        carbon dioxide absorbents of the present invention. Typical        chemical absorption behaviour is observed at low CO₂ pressure,        with the CO₂ uptake increasing asymptotically as the 1:1 molar        ratio is approached. Once the saturation pressure is reached        (i.e. a 1:1 molar ratio of ionic liquid and CO₂), the system        switches to the linear response expected of a physical CO₂        absorber. The linear increase in absorption is observed        continuously to the highest CO₂ partial pressure observed.    -   FIG. 2 is a histogram showing a comparison of the CO₂ absorption        of various CO₂ absorbents from Examples 3 to 7.

EXAMPLES

General

This example describes the general experimental method used to determinethe solubility of carbon dioxide in the ionic liquid-water mixtures.

In a typical experiment, the volume of a pressure vessel [Parr pressuresystem] is first determined by evacuating it under reduced pressure andsubsequently pumping a known amount of gas at a certain temperature andpressure into the vessel. Measurement of the amount of gas is read asthe volume of gas at standard conditions from the mass flow controller[BROOKS Smart Massflow]. The ideal gas law is used to calculate theactual volume of the pressure vessel.

A known mass and volume of an ionic liquid-water mixture is placed in apressure vessel and degassed for 5 min under reduced pressure. Carbondioxide is then pumped into the stirred pressure vessel (1000 rpm)through the mass flow controller up to 500 kPa and at 25.0° C. Thesystem is allowed to equilibrate for 15 min or until no more gas isadded according to the mass flow controller.

Calculation of the total amount of gas introduced in the pressure vesselis made using the reading in the mass flow controller. The actual amountof gas in the gas phase is calculated by the ideal gas law, where thevolume of the gas phase was equal to the volume of the pressure vesselminus the volume of the liquid phase. The amount of gas dissolved in theliquid phase was calculated by subtracting the actual amount of gas inthe gas phase from the total amount of gas introduced into the pressurevessel.

Results are expressed as a molar concentrations by weight(mol(CO₂)/kg(ionic liquid)) and by volume (mol(CO₂)/L(ionic liquid)).Ratios of ionic liquid to water are molar ratios. The amount of water inthe liquid mixtures was quantified by Karl-Fischer titration, and/or ¹HNMR.

Example 1 (Comparative Example)

Solubility of Carbon Dioxide in TributyloctylphosphoniumDibutylphosphate

The solubility of carbon dioxide in ionic liquids containing anionswhich are conjugate bases of acids having a pKa of less than 3.6 wasexamined using tributyloctylphosphonium dibutylphosphate([P_(4,4,4,8)][DBP]), which has the following formula:

Carbon dioxide absorption by this ionic liquid was examined using boththe neat ionic liquid and a 1:1 molar ratio of the ionic liquid andwater. The results are shown in Table 1

TABLE 1 Mass mol(CO₂)/ mol(CO₂)/ Ionic liquid (IL) IL (g) L(IL) kg(IL)[P_(4,4,4,8)][DBP] 6.011 0.3337* 0.3526* [P_(4,4,4,8)][DBP]/H₂O (1:1)5.477 0.4639 0.4878 *Average over three runs

These results show that the ionic liquid does not dissolve as much CO₂as expected. In particular, the presence of 1 molar equivalent of wateronly gives rise to a slight increase in the solubility of carbon dioxidein the ionic liquid. It is believed that the acid/base reaction of theionic liquid with carbonic acid does not take place due to the low pKaof the conjugate acid of the [DBP] anion, (pKa=ca. 1.72).

Example 2 (Comparative Example)

Solubility of Carbon Dioxide in TetrabutylphosphoniumDihydrogenphosphate

Tetrabutylphosphonium dihydrogenphosphate ([P_(4,4,4,4)][H₂PO₄]) wasproduced by titrating tetrabutylphosphonium hydroxide with phosphoricacid to the pH end point of 4.54 with the assistance of a pH meterbefore removal of water under reduced pressure. The resulting productwas a solid at room temperature, but readily liquefied when combinedwith water at an ionic liquid to water molar ratio of 1:4. Carbondioxide with this ionic liquid was examined according to the proceduredescribed above, and the results are shown in Table 2.

TABLE 2 Mass mol(CO₂)/ mol(CO₂)/ Ionic liquid (IL) IL (g) L(IL) kg(IL)[P_(4,4,4,4)][H₂PO₄] N/A* [P_(4,4,4,4)][H₂PO₄]/H₂O (1:1) 5.494 0.4840.468 *Solid at the operating temperature of the process - absorptioncould not be measured

These results again show that the ionic liquid does not dissolve as muchCO₂ as expected. It is believed that the acid/base reaction of the ionicliquid with carbonic acid does not take place due to the low pKa ofphosphoric acid, (pKa=ca. 2.12).

Example 3

Solubility of Carbon Dioxide in Monohydrogenphosphate Ionic Liquids

The ionic liquids tetrabutylphosphonium hydrogenphosphate([P_(4,4,4,4)]₂[HPO₄]), tetraethyl-ammonium hydrogenphosphate([N_(2,2,2,2)]₂[HPO₄]), tributylmethylammonium hydrogen-phosphate([N_(1,4,4,4)]₂[HPO₄]), tetrapropylammonium hydrogenphosphate(N_(3,3,3,3)][HPO₄]), and tetrabutylammonium hydrogenphosphate(N_(4,4,4,4)][HPO₄]) were produced by titration of the correspondinghydroxides with phosphoric acid to a pH end point of 9.28 with theassistance of a pH meter. The ionic liquids were used in two forms: (i)“wet” wherein the ionic liquid was used in the form obtained by dryingusing a rotary evaporator without the application of vacuum, such thatthe water content is not precisely defined but is still thought to bepresent in molar excess relative to the ionic liquid; and (ii) indefined molar ratios of ionic liquid to water, by removing water todryness under high vacuum and then combining the ionic liquid residuethe required volume of water. The results are shown in Table 3.

TABLE 2 Mass mol(CO₂)/ mol(CO₂)/ Ionic liquid (IL) IL (g) L(IL) kg(IL)[P_(4,4,4,4)]₂[HPO₄]/H₂O (1:1) 6.943 1.804 1.808 [P_(4,4,4,4)]₂[HPO₄](wet) 6.610 2.060 2.007 [N_(2,2,2,2)]₂[HPO₄]/H₂O (1:5) 5.602 1.906 1.663[N_(1,4,4,4)]₂[HPO₄]/H₂O (1:5) 5.393 2.459 2.393 [N_(1,4,4,4)]₂[HPO₄](wet) 8.665 2.299 2.237 [N_(3,3,3,3)][HPO₄]/H₂O (1:3) 1.518[N_(4,4,4,4)][HPO₄]/H₂O (1:4) 1.434

These results demonstrate that the solubility of carbon dioxide in ionicliquid-water mixtures increases remarkably when the ionic liquid anionis the conjugate base of an acid having a pKa above 3.60 (pKa[H₂PO₄]=7.21).

Example 4 (Comparative Example)

Solubility of Carbon Dioxide in Neat 1-hexyl-3-methylimidazoliumbis(trifluoromethane)-sulfonimide) ([C₆mim][NTf₂])

The solubility of carbon dioxide in neat [C₆mim][NTf₂] was measured at500 kPa and at 25.0° C. as described in Example 1. This liquid solutionwas chosen as a comparative example of an ionic liquid CO₂ absorberhaving an anion which is a conjugate base of an acid having a pKa ofless than 3.6. The solubility of carbon dioxide in this ionic liquid was0.52 mol˜L⁻¹.

Example 5 (Comparative Example)

Solubility of Carbon Dioxide in TetrabutylphosphoniumDihydrogenphosphate ([P_(4,4,4,4)][H₂PO₄]) and Water

The solubility of carbon dioxide in neat [P_(4,4,4,4)][H₂PO₄] and water(1:4 molar ratio) was measured at 500 kPa and at 25.0° C. as describedin Example 1. This liquid solution was chosen as a comparative exampleof an ionic liquid CO₂ absorber comprising water and having an anionwhich is a conjugate base of an acid having a pKa of less than 3.6. Thesolubility of carbon dioxide in this ionic liquid was 0.484 mol·L⁻¹.

Example 6 (Comparative Example)

Solubility of Carbon Dioxide in 1M Aqueous Na₂HPO₄

The solubility of carbon dioxide in neat [C₆mim][NTf₂] was measured at500 kPa and at 25.0° C. as described in Example 1. This liquid solutionwas chosen as a comparative example of an aqueous CO₂ absorber having ananion which is a conjugate base of an acid having a pKa of greater than3.6, but which has a sodium cation as the counterion instead of an ionicliquid cation. The solubility of carbon dioxide in this ionic liquid was0.49 mol·L⁻¹.

Example 7 (Comparative Example)

Solubility of Carbon Dioxide in Monoethanolamine/water Mixtures

The solubility of carbon dioxide in a mixture of monoethanolamine (MEA)and water (30:70 MEA/H₂O weight ratio) was measured at 500 kPa and at25.0° C. as described in Example 1. This liquid solution was chosen as acomparative example of a carbon dioxide chemical absorber usedcommercially in industry, especially in natural gas processingoperations. The solubility of carbon dioxide in themonoethanolamine/water mixture was found to be 3.57 mol·L⁻¹. It willtherefore be appreciated that the ionic liquid-water mixtures of thepresent invention clearly provide viable alternatives to theconventional use of monoethanolamine/water mixtures.

CONCLUSION

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

The invention claimed is:
 1. A process for removing carbon dioxide froma gaseous stream comprising contacting the gaseous stream with a carbondioxide absorbent comprising a mixture of an ionic liquid and water in amolar ratio of from 10:1 to 1:10, wherein the ionic liquid has theformula:[Cat⁺][X⁻] wherein: [Cat⁺] represents one or more cationic speciesselected from tetrasubstituted phosphonium cations, tetrasubstitutedammonium cations, trisubstituted sulfonium cations, guanidinium cationsand quinolinium cations; and [X⁻] represents one or more anionic speciesselected from conjugate bases of acids having a pKa of at least 3.6,where the one or more anionic species includes at least one of phosphatedianions, phosponate dianions, or dicarboxylate dianions, with theproviso that [X⁻] is not formate, acetate, trifluoroacetate,hydroxyacetate, propanoate, pentafluoro-propanoate, lactate, butanoate,isobutanoate, pivalate, pyruvate, thiolactate, oxalate, tartrate,malonate, succinate, adipate or benzoate.
 2. A process according toclaim 1, wherein [Cat⁺] is selected from:[P(R^(a))(R^(b))(R^(c))(R^(d))]⁺,[N(R^(a))(R^(b))(R^(c))(R^(d))]⁺ and[S(R^(b))(R^(c))(R^(d))]⁺ wherein R^(a), R^(b), R^(c), and R^(d) areeach independently selected from a C₁ to C₂₀ straight chain or branchedalkyl group, a C₃ to C₈ cycloalkyl group, or a C₆ to C₁₀ aryl group, orwherein any two of R^(a), R^(b), R^(c), and R^(d) may together form asaturated methylene chain of the formula —(CH₂)_(q)—, where q is aninteger of from 4 to 7, or an oxyalkylene chain of the formula—(CH₂)₂—O—(CH₂)₂—, wherein said alkyl, cycloalkyl or aryl groups, saidmethylene chain, or said oxyalkylene chain are unsubstituted or may besubstituted by one to three groups selected from: C₁ to C₆ alkoxy, C₂ toC₁₂ alkoxyalkoxy, C₆ to C₁₀ aryl, —CN, —OH, —NO₂, —CO₂(C₁ to C₆)alkyl,—OC(O)(C₁ to C₆)alkyl, C₇ to C₃₀ aralkyl C₇ to C₃₀ alkaryl, and—N(R^(z))₂, where each R^(z) is independently selected from hydrogen,methyl, ethyl, n-propyl and iso-propyl, and wherein R^(b) may also behydrogen.
 3. A process according to claim 2, wherein [Cat⁺] is selectedfrom:[P(R^(a))(R^(b))(R^(c))(R^(d))]⁺ and [N(R^(a))(R^(b))(R^(c))(R^(d))]⁺wherein: R^(a), R^(b), R^(c), and R^(d) are as defined in claim
 2. 4. Aprocess according to claim 3, wherein [Cat⁺] is selected from:[P(R^(a))(R^(b))(R^(c))(R^(d))]⁺ wherein: R^(a), R^(b), R^(c), and R^(d)are as defined in claim
 2. 5. A process according claim 2, whereinR^(a), R^(b), R^(c), and R^(d) are each independently selected, wherepresent, from a C₁ to C₁₆ straight chain or branched alkyl group, orwherein any two of R^(a), R^(b), R^(c), and R^(d) together form amethylene chain of the formula —(CH₂)_(q)—, where q is an integer offrom 4 or
 5. 6. A process according to claim 1, wherein [Cat⁺] isselected from quinolinium cations of the formula:

wherein: R^(a), R^(b), R^(c), R^(d), R^(e), R^(f), R^(g), R^(h) andR^(i) are each independently selected from hydrogen, a C₁ to C₂₀straight chain or branched alkyl group, a C₃ to C₈ cycloalkyl group, ora C₆ to C₁₀ aryl group, or any two of R^(b), R^(c), R^(d), R^(e), R^(f),R^(h) and R^(i) attached to adjacent carbon atoms may form a saturatedmethylene chain —(CH₂)_(q)— wherein q is from 3 to 6, and wherein saidalkyl, cycloalkyl or aryl groups, or said methylene chain areunsubstituted or may be substituted by one to three groups selectedfrom: C₁ to C₆ alkoxy, C₂ to C₁₂ alkoxyalkoxy, C₆ to C₁₀ aryl, —CN, —OH,—NO₂, —CO₂(C₁ to C₆)alkyl, —OC(O)(C₁ to C₆)alkyl, C₇ to C₃₀ aralkyl C₇to C₃₀ alkaryl, and —N(R^(z))₂, where each R^(z) is independentlyselected from hydrogen, methyl, ethyl, n-propyl and iso-propyl.
 7. Aprocess according to claim 1, wherein [Cat⁺] is selected fromguanidinium cations of the formula:

wherein: R^(a), R^(b), R^(c), R^(d), R^(e), and R^(f) are eachindependently selected from a C₁ to C₂₀ straight chain or branched alkylgroup, a C₃ to C₈ cycloalkyl group, or a C₆ to C₁₀ aryl group, or anytwo of R^(b), R^(c), R^(d), R^(e), R^(f), R^(h) and R^(i) attached toadjacent carbon atoms may form a saturated methylene chain —(CH₂)_(q)—wherein q is from 3 to 6, and wherein said alkyl, cycloalkyl or arylgroups, or said methylene chain are unsubstituted or may be substitutedby one to three groups selected from: C₁ to C₆ alkoxy, C₂ to C₁₂alkoxyalkoxy, C₆ to C₁₀ aryl, —CN, —OH, —NO₂, —CO₂(C₁ to C₆)alkyl,—OC(O)(C₁ to C₆)alkyl, C₇ to C₃₀ aralkyl C₇ to C₃₀ alkaryl, and—N(R^(z))₂, where each R^(z) is independently selected from hydrogen,methyl, ethyl, n-propyl and iso-propyl.
 8. A process according to claim1, wherein [X⁻] represents an anionic species which is a conjugate baseof an acid having a pKa of 4.0 or more.
 9. A process according to claim1, wherein [X⁻] represents an anionic species which is a conjugate baseof an acid having a pKa of 15.0 or less.
 10. A process according toclaim 1, wherein [X⁻] is selected from phosphate anions having theformula [R^(x)OP(O)O₂]²⁻ and phosphonate anions having the formula[R^(x)P(O)O₂]²⁻, wherein R^(x) is selected from hydrogen, a C₁ to C₁₀straight chain or branched alkyl group, a C₃ to C₈ cycloalkyl group, aC₆ to C₁₀ aryl group, a C₆ to C₁₄ aralkyl group, or a C₆ to C₁₄ alkarylgroup, wherein said alkyl, cycloalkyl, aryl, aralkyl, or alkaryl groupsare unsubstituted or may be substituted by one or more groups selectedfrom —F, —Cl, —Br, —I , —OH, —CN, —NO₂, —SH, and ═O.
 11. A processaccording to claim 10, wherein [X⁻] is selected from phosphate anionshaving the formula [R^(x)OP(O)O₂]²⁻, where R^(x) is as defined in claim10.
 12. A process according to claim 10, wherein R^(x) is selected fromhydrogen or a C₁ to C₁₀ straight chain or branched alkyl group, whereinsaid alkyl group is optionally substituted by one or more groupsselected from —F, —Cl, —Br, —I , and —OH.
 13. A process according toclaim 12, wherein [X⁻] is [HOP(O)O₂]²⁻.
 14. A process according to claim8, wherein [X⁻] is selected from dicarboxylate dianions having theformula [O₂C—R^(y)—CO₂]²⁻, wherein R^(y) represents a C₂ to C₆ straightchain or branched alkylene or alkenylene chain, a C₁ to C₆ cycloalkylenegroup, or a C₆ arylene group, wherein said alkylene, alkenylene,cycloalkylene or arylene groups are unsubstituted or may be substitutedwith one or more groups selected from —F, —Cl, —Br, —I , —OH, —CN, —NO₂,—SH, —CO₂H and ═O, with the proviso that [X⁻] is not succinate oradipate dianion.
 15. A process according to claim 14, wherein [X⁻] is adicarboxylate dianion selected from glutarate dianion, pimelate dianion,methylmalonate dianion, fumarate dianion, maleate dianion, methylsuccinate dianion, malate dianion, citrate dianion, itaconate dianion,and mesaconate dianion, o-phthalate dianion, m-phthalate dianion,p-pthalate dianion, aspartate dianion, glutamate dianion, octanedioicacid dianion, and heptanedioic acid dianion.
 16. A process according toclaim 1, with the proviso that [X⁻] does not comprise a conjugate baseof a monocarboxylic acid.
 17. A process according to claim 1, with theproviso that [X⁻] does not comprise a conjugate base of a carboxylicacid.
 18. A process according to claim 1, wherein the ionic liquid has amelting point of 200° C. or less.
 19. A process according to claim 1,wherein the mixture of an ionic liquid and water has a melting point of100° C. or less.
 20. A process according to claim 1, wherein the molarratio of ionic liquid to water is from 5:1 to 1:10.
 21. A processaccording to claim 1, wherein the gaseous stream is contacted with thecarbon dioxide absorbent at a temperature of from 10 to 80° C.
 22. Aprocess according to claim 1, wherein the gaseous stream is contactedwith the carbon dioxide absorbent at a pressure of from 100 to 5000 kPa.23. A process according to claim 1, wherein carbon dioxide issubsequently released from the carbon dioxide absorbent.
 24. A processaccording to claim 23, wherein the carbon dioxide is subsequentlyreleased by subjecting the carbon dioxide absorbent to reduced pressure,or by sparging the carbon dioxide absorbent with a gas at elevatedtemperature.
 25. A process according to claim 1, wherein the gaseousstream is a hydrocarbon-containing gaseous stream.
 26. A processaccording to claim 25, wherein the gaseous stream is amethane-containing gaseous stream.
 27. A process according to claim 26,wherein the gaseous stream is a natural gas stream.
 28. A processaccording to claim 26, wherein the gaseous stream is a biogas-derivedstream.
 29. A process according to claim 1, wherein the gaseous streamis a flue gas stream.
 30. A process according to claim 1, wherein [X—]comprises [HOP(O)O₂]²⁻.